1. Field of the Invention
The present invention is related to microelectronic process technology, and more particularly to Micro- and Nano-Electro Mechanical Systems (MEMS and NEMS) process technology. The present invention relates to a method for forming a hermetically sealed cavity, for example a cavity comprising a MEMS or a NEMS device. Embodiments also relate to a wafer level encapsulation method for MEMS and/or NEMS devices.
2. Description of the Related Technology
Many micro-electromechanical systems (MEMS) require a hermetic encapsulation under vacuum or under a controlled atmosphere and pressure in order to ensure a good performance and/or an acceptable lifetime of operation. The encapsulation has to be performed without influencing the device performance. This often requires low-temperature processing, the absence of any aggressive or corrosive agents during encapsulation and sealing, and avoidance of deposition of sealing material onto the MEMS device, as all these can cause damage to the device.
Two approaches for device encapsulation can be defined. A first approach is encapsulation during first level packaging. However, when following this approach the fragile, often freely movable, MEMS device is exposed to contamination during wafer dicing and subsequent cleaning. A second approach is zero level packaging or wafer level packaging, whereby the package is designed and fabricated at the same time as the MEMS device. Zero level packaging may be obtained by means of wafer bonding (e.g. wafer-to-wafer or die-to-wafer bonding) or may be based on surface micromachining techniques.
The most popular approach is based on wafer bonding. In this case, sealing is performed by connecting a device wafer and a capping wafer (or a capping die) in which a cavity is made or a stand-off ring is implemented, by use of a reflowable material. In the wafer bonding approach a substrate such as a Si wafer or a MEMS substrate, is used as a cap to close the cavity comprising the MEMS device. These wafers are thick and the sealing ring is large. Therefore, this packaging approach is space consuming. Moreover, batch processing is not possible.
A more compact way of hermetic sealing of a MEMS device is to make use of thin-film caps realized by surface micromachining. In this approach, access holes or channels may be formed in the thin-film caps which allow passing through of e.g. a MEMS sacrificial layer etchant for etching away a sacrificial layer for forming a cavity. After forming a cavity and, optionally release of a MEMS device present in the cavity, the channel may be closed, thereby sealing the cavity.
In EP 1 433 741 a zero-level packaging method is described based on closing an opening or openings in a membrane using a reflow material, the openings being located above an underlying cavity in a substrate. The method comprises depositing an intermediate layer onto the membrane layer whereby this intermediate layer narrows down the openings to be sealed. Next, a reflow layer is deposited on the intermediate layer under a first set of pressure and atmosphere conditions to further partially close the openings. Then, the reflow layer is reflowed under a second set of pressure and atmosphere conditions to close the openings by the reflow layer. The intermediate layer prevents reaction between the sealing layer and the membrane layer, is stable during reflow and narrows down the openings to be sealed due to the formation of a collar or shoulder. This method allows hermetic sealing of openings in a film at controllable atmosphere and pressure. When using this method, there is, however, a risk that material of the different deposited layers passes through the openings and that this material is deposited on fragile micro-devices, such as MEMS devices, that may be located in the cavity under the openings, thereby affecting proper working of such micro-devices.
In the above-described method process temperatures are typically higher than 600° C. A method for hermetically sealing cavities at a lower temperature is disclosed in U.S. Pat. No. 7,029,829. This document describes a low-temperature method for forming a micro-cavity on a substrate and is particularly useful to package MEMS devices in vacuum on the wafer level and to provide sealed feed-throughs to the outside world. According to this method, the temperature may not exceed 250° C. during the entire method. In this method, the opening or openings to be sealed are not located above the underlying cavity, but large side ports, also called “horizontal” access holes, are used. Because the opening or openings which are used for releasing sacrificial material to form the cavity are not located above the cavity, there is never a risk that the MEMS device in the cavity would be harmed by the materials used passing through the holes and being deposited onto the MEMS device in the cavity. However, a disadvantage of these side ports or horizontal access holes is that release times of several hours are required for forming the cavity.
In WO 2005/089348 a method is described for packaging MEMS devices at low temperatures (200° C. to 300° C.), wherein no access holes are used. In this process an overcoat layer is deposited on a sacrificial polymer layer that is removed prior to package sealing. Removing the sacrificial polymer layer is based on thermal decomposition, whereby the decomposition products of the sacrificial layer diffuse through the overcoat layer. This results in release times of several hours.